The differential thermal analysis is used in order to be able to draw conclusions on the composition of materials, such as metals and metal alloys among others. It supplies substantially more comprehensive and clearer statements concerning the phase composition of the tested material than the normal thermal analysis (evaluation of time temperature curves). The method itself is carried out in laboratory conditions for test samples with a low microgram mass of up to 10 g. The differential thermal analysis cannot be carried out with conventional DTA (Differential-Thermo-Analysis) devices on test samples with a mass of over 10 g which must generally be used for a quick operational control of metal melts. Neither can the classical differential thermal analysis supply any statements concerning solidification behavior and phase composition of real castings.
In another known, but not used, method of the differential thermal analysis the cooling curve of the test sample is compared with the curve of a group of fixedly given curves which satisfy Newton's law of cooling U=U.sub.O .times.e.sup.-t/RC, U.sub.O corresponding to the maximum value of the temperature at the point of time t=0 and RC being the time constant of the curve. The fixedly given curves are under some circumstances empirically determined taking into consideration all influencing factors (change in material constants and heat transfer conditions during cooling of the test sample to be tested) and stored by computer from which they can be called dependent on the expected cooling curve of the test sample.
In such a method of differential thermal analysis, none of the curves of the group generally coincides fully with the cooling curve of the test sample in the liquid zone. For this reason, this method for metals and metal alloys and, in particular, cast iron and steel, is inaccurate even when interpolation is made between given curves to adapt them to the cooling curve of the test sample.
In another known process an imaginary connecting line between the first and last (at the end) transformations of the test sample, which are characterized by thermal effects, is used as a comparative curve. All the results calculated on the basis of such an approximate curve are affected with systematic faults.
In a known method of the aforementioned type the cooling curve is measured at a fixed test sample with a thermal element. The course of cooling of a fixed test sample, however, does not show any distinct heat effects. It has been attempted to increase the cooling speed in order to compensate this disadvantage. An increased cooling speed, however, is accompanied by difficulties in adjusting the comparative curve. The conclusions in the case of this method are also invalidated by using the thermal element as the measuring detector. That is, thermal elements falsely record the course of cooling of the test sample due to disturbing proximity influences.
The object of the invention is to provide a method of the aforementioned type and an apparatus suited to carrying out the method, both of which are simpler to use and supply more exact differential values than known methods and apparatus.
This object is solved according to the invention in that by using a vessel for molten metal, the cooling curve of the molten test sample is measured on the metal/vessel boundary layer and these measurements are used for adjusting the parameters of the comparative curve.
Very exact difference values are obtained with the method of the invention since the heat effects above all in the molten zone of the curve at the metal/vessel boundary layer are the least inaccurate. The accuracy of the comparison test sample to be adjusted can be further improved by the determined difference values being differentiated.
An apparatus to carry out the method comprises a temperature measuring device and a comparator which records the deviation of the cooling curve of the test sample from a fixedly given comparative curve which is produced by a signal transmitter working according to Newton's law (U=U.sub.O .times.e.sup.-t/RC, the U.sub.O parameter being the maximum value at the point in time t=0 and the RC parameter being the time constant), to which an adjustment means is arranged which, dependent on the parameters determining the cooling curve of the test sample in one curve section, adjusts the corresponding parameters U.sub.O and RC of the comparative curve for the corresponding curve section at the signal transmitter in such a manner that the two curve sections coincide, and is characterized in that the measuring detector of the temperature measuring device includes a light conductor which closes with its front side flush with the inside of the mold hollow space of a casting mold for the molten test sample.
The comparator is preferably constructed in the form of a summation instrument for the deviation of the two curves. In addition, a differentiator can be arranged after the summation instrument.
According to a further embodiment of the invention an elastic bonding agent can be arranged between the light conductor and the casting mold. This bonding agent should on the one hand oppose the ferro-static pressure by sufficient resistance to displacement of the light conductor, but on the other hand not hinder the movement of the light conductor in contraction direction of the metal on solidification and cooling. It has namely been shown that the front side of the light conductor adheres to the surface of the solidified metal block most probably caused by early formation of a narrow metal ridge on the light conductor/casting mold boundary surface which shrinks onto the light conductor and communicates hereto the movements of the metal wall.
According to a further embodiment the light conductor is composed of optical quartz glass for temperatures over 500.degree. C.
The invention is explained below in further detail by means of a drawing representing an embodiment.